Young mothers may be more likely to have unprotected sex in encounters with the fathers of their children, increasing their risk of repeat pregnancies and sexually transmitted diseases, a U.S. study suggests.

Researchers followed 171 women ages 15 to 24 in Baltimore, Maryland who had at least one sexual partner after becoming mothers.

When the women had sex in what they colloquially called "baby daddy" relationships, they were more than 12 times more likely to skip condoms for vaginal sex and more than thee times as likely to have unprotected anal intercourse.

"We have heard qualitatively for years that condom use is minimal in sexual relationships where there is a shared child," said lead researcher Michele Decker of Johns Hopkins University in Baltimore, in an email.

And indeed, she added, in this study, "condom nonuse for both vaginal and anal intercourse was more common in those relationships."

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To understand how shared children may influence sexual behavior, Decker and colleagues analyzed survey data collected from 2011 to 2013 from heterosexual men and women who were sexually active.

Participants were asked whether they had casual or more committed sexual relationships, what condoms or other contraceptives they used, and whether they thought their partners might have a sexually transmitted disease or risk factors such as a history of drug use, gang involvement or incarceration.

One in four of the women reported at least one "baby daddy" partner, and the male partners in these relationships were more likely to have been incarcerated or arrested, the study found. These men were also more likely to be described by the women as a primary rather than a casual relationship.

The men who had children with the women also tended to be more likely to have a sexually transmitted disease, a history of using intravenous drugs or dealing, and prior gang involvement - but all of these differences were small and might have been due to chance.

The findings suggest that young mothers who have sex with the fathers of their children may be at increased risk for sexually transmitted diseases and unplanned pregnancies, the researchers conclude in the journal Sexually Transmitted Infections.

Limitations of the study include the lack of data on co-habitation or marriage status among the couples, as well as information on how the male partners characterized their relationships with the women, the authors acknowledge.

It's also possible that the main reason women agreed to unprotected sex might have more to do with their perception of being in a serious relationship with the men who fathered their children, particularly if the couples were married or living together, noted Renee Sieving, a nursing and pediatrics researcher at the University of Minnesota in Minneapolis.

"Young women are less likely to use condoms with main/steady partners than with casual partners," Sieving, who wasn't involved in the study, said by email.

Previous research has also shown that biological fathers may expect to have unprotected sex with these women because they've done it before, said Dr. Elizabeth Miller, chief of adolescent and young adult medicine at the Children's Hospital of Pittsburgh of UPMC.

"Fathers of young women's babies may have a sense that even if they are no longer officially together that they can return to the baby's momma any time for sex," Miller, who wasn't involved in the study, said by email.